The present invention relates to an improved turbomolecular pump, especially to a turbomolecular pump of increased compression ratio, capable of extending the operating range towards higher pressures.
Conventional turbomolecular pumps usually have large operating ranges from about 10.sup.-7 to 10.sup.-1 or 1 Pascal, however they cannot exhaust directly to atmosphere. This means that they need to be teamed up to a forepump which produces the necessary fore vacuum to discharges the pumped gases at atmospheric pressure. However, contamination of the turbomolecular pump with lubrication oil of the forepump may occur, which prevents pumping at the lower operating range. This may be avoided by maintenance at short intervals, which raises the costs of operation, in addition to a high initial cost of the vacuum system. Moreover, the combination of a turbomolecular pump with a forepump is cumbersome, which is a disadvantage in most applications.
So-called hybrid turbomolecular pumps have also been developed to reduce the necessity of these backing pumps. U.S. Pat. No. 4,732,529, U.S. Pat. No. 4,826,393 and U.S. Pat. No. 4,797,068, disclose turbomolecular pumps including a compression ratio raising section consisting of rotors formed with spiral grooves, or screw rotors, which guide gas from the high vacuum section to a simpler exhaustion system, e.g., to a membrane pump. Although such hybrid turbomolecular pumps do not need complex exhaustion systems consisting of a number of auxiliary vacuum pumps, they still require a forepump, because they are incapable of discharging gases at atmospheric pressure.
A new type of hydrid roughing pump is reported which can reach a roughing pump level of low ultimate pressure (3.times.10.sup.-2 Pascal) J.Vac.Sci.Technol., A, Vol. 6, No. 4, pp. 2518-21, July/August 1988. This reported pump is a turbo vacuum roughing pump comprising radial flow pumping stages consisting of impellers rotating into channels with grooves which direct radially the flow of the pumped gases, and a peripheral flow pumping stage at the exhaust side, which raises the pressure so that the pump can discharge at atmospheric pressure. However, this pump is only a roughing pump that can not replace a turbomolecular pump, the ultimate pressure of which is lower by several orders of magnitudes (10.sup.-7 Pascal) than the ultimate pressure of this roughing pump (10.sup.-2 Pascal).
A first object of the present invention is to provide a new hybrid turbomolecular pump with a high compression ratio.
Another object of the present invention is to provide a new hybrid turbomolecular pump which is capable of discharging gases at atmospheric pressure, without being combined with a forepump.
A further object of the present invention is to provide a turbomolecular pump which is relatively simple in comparison with previous vacuum systems having similar operating range.